It sounds you have a +R dvd recorder, some of them can only burn one kind of disk.

Make sure when you write a disk that the speed you use in nero is the same as the speed of the disk, so if you have a 16x speed disk, don`t make the write speed higher than 16x or it is likely to cause errors (you can safely write at a slower speed than the disk write speed if you want to for some reason).